In a gas and steam turbine plant, the exhaust gas exiting a gas turbine is used for generating steam for the steam turbine. The heat transfer takes place in this case in a waste-heat steam generator which is arranged downstream of the gas turbine and in which there are arranged, in an exhaust-gas channel, a number of heating surfaces for preheating feed water, for generating steam and also for subsequently superheating steam. For this purpose, the heating surfaces are connected into the feed water/steam circuit of the steam turbine and are thus flowed through one after the other by the medium flowing in said circuit.
For the waste-heat steam generator arranged downstream of the gas turbine on the exhaust-gas side, a number of alternative design concepts come into consideration, namely the design as a once-through steam generator or the design as a circulation steam generator. In a once-through steam generator, the heating of evaporator pipes of the evaporator heating surfaces leads to evaporation of the flow medium in a single pass.
When starting up the waste-heat steam generator, a so-called water ejection can occur. This arises when the evaporation, occurring as a consequence of the heating of the evaporator pipes which begins after the start, of the flow medium present in said pipes happens for the first time. If this takes place, for example, in the center of the respective evaporator pipe, the quantity of water present downstream (also referred to as the “water plug”) is pushed out of the respective evaporator pipe. In order to safely rule out the possibility of non-evaporated flow medium passing from the evaporator pipes into the downstream superheater heating surfaces, a water separator is normally provided between the evaporator heating surfaces and the downstream superheater heating surfaces. In said water separator, separated water is then fed to an expansion device. The steam formed during the expansion in the atmospheric expansion device is normally discharged to the surroundings, and this contributes significantly to water loss during the start-up of the waste-heat boiler. The water which accumulates during the expansion in the atmospheric expander can be fed again into the feed water/steam circuit. The water guided into the atmospheric expander increases the energy losses during the start-up of the waste-heat steam generator.
If such water ejection during the start-up is now to be avoided to the maximum possible extent, an obvious solution would consist in a valve being arranged downstream of the preheater heating surfaces, such that it is possible to regulate the feed water quantity flowing into the downstream evaporator heating surfaces. However, the preheater system must in this case be designed for a relatively high pump pressure, and this leads to additional costs.